Interfaces, unions, and generics in practice
By the end of today you can model states so that impossible combinations cannot be expressed, narrow a union safely, and read a generic signature without flinching.
YesterdayOn Day 29 you learned what the checker does. Today you use the parts that actually prevent bugs.
TomorrowTomorrow you take these shapes into the browser, where state has somewhere to live.
Why this matters
Modelling data so invalid states are unrepresentable removes whole classes of bug before they exist. It is the same instinct as a database constraint on Day 44 and a tool schema on Day 78.
- Interfaces and type aliases
- Unions and impossible states
- Narrowing
- Generics
Learn it
75 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
Free · sign inA prompt written for this day alone: your level, the exact scope, what to leave out, and an instruction to quiz you before it explains anything. Paste it into Claude or ChatGPT and it teaches you today's material.
Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
4 hand-picked resources
Free · sign inVideos, official docs and articles covering the same ground, each opened and annotated by hand. They are what you check the assistant against on a day you cannot yet catch it being wrong.
Build it
45 minModel the state of a data fetch two ways: first with three independent optional fields, then as a discriminated union. Write a function that renders each state and add an exhaustiveness check. Then add a fourth state and confirm the build breaks in exactly the right place. Finally, write one small generic function and use it with two different types.
Recall it
20 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
5 recall questions
Free · sign inQuestions you answer from memory, then grade yourself against the real answer. The score is carried into the mastery rating below it, so an honest miss cannot quietly become a tick.
Rate it
Completion and mastery are tracked separately. Be honest, because an inflated rating only means the concept resurfaces sooner.
Mastery tracking
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Recap
- 01Model so that impossible states cannot be written down
- 02Narrowing is a runtime check the compiler can follow
- 03never turns a new case into a build error instead of a silent bug
- 04A generic is just a type the caller supplies
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.